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1 Coulomb Counter Only Option 2 Coulomb Counter With Discharge and Temperature Compensation Option Application Report SLUA367–December 2005 Using the bq27x00 With a Primary Cell Battery Bill Jackson ................................................................................................................ Portable Power ABSTRACT The bq27x00 contains an accurate coulomb counter and has algorithms to determine the available capacity and to dynamically compensate the available capacity for temperature and discharge rate. If direct drive from the gauge to an LED display is not required, the bq27x00 can be used as a complete gas gauge solution for a primary cell battery. Although the bq27x00 was designed for a single-cell rechargeable lithium-ion or lithium-polymer battery, it can be used for any primary cell chemistry because charging efficiency is not important and algorithms specific to the lithium chemistry are not needed. It also can be used with a multicell battery with the addition of an external LDO to provide a regulated Vcc and a voltage divider or active attenuator to provide voltage to the BAT input. The bq27x00 automatically enters low-power sleep mode if a communication line is low for at least 20 seconds. The gauge can provide time-to-empty (TTE) in minutes, nominal available charge (NAC), relative state-of-charge (RSOC), and many other parameters. Used as a coulomb counter, the bq27x00 simply measures the capacity removed from the battery by using the initial last measured discharge (ILMD) value as the design capacity of the battery. The ILMD value should be programmed for the minimum available capacity from the battery under worst-case operating conditions. The minimum available capacity should consider both maximum load and minimum temperature operational conditions. The disadvantage of this approach is that the minimum available capacity may be severely reduced at cold temperature and thus would prevent the ability to obtain longer run times when the system is not used in a cold temperature environment. If temperature compensation (TCOMP) and discharge compensation (DCOMP) values are programmed to reduce the available capacity at cold temperature and high discharge rates, the system is able to achieve longer run times when used in a typical operational scenario. ILMD can be programmed for the minimum available capacity for the battery with a minimum operating load and a nominal operating temperature and TCOMP and DCOMP values can be programmed for the appropriate temperature and discharge rate capacity compensation values to dynamically adjust the remaining capacity prediction according to the actual use environment. The compensated available capacity with temperature (and discharge rate), or CACT, is reduced below the NAC value at high discharge rates according to the programmed DCOMP parameter. If the discharge rate is later reduced so that there is less discharge rate compensation, the CACT value remains depressed until NAC decreases to the point where the NAC and CACT difference matches the capacity reduction at the lower load current. CACT is also reduced below NAC at cold temperature according to the programmed TCOMP parameter. However, the CACT reduction due to temperature is not held and CACT may rise immediately with a temperature increase. The temperature compensation calculation is independent of load current; so, the CACT compensation coefficient should be chosen to SLUA367–December 2005 Using the bq27x00 With a Primary Cell Battery 1